1. Field of the Invention
The present invention relates to laser marking for irradiating a material to be printed such as a photosensitive material with a laser beam and forming a predetermined marking pattern such as a character, a sign, or the like by dots which can be visually recognized on the material to be printed. More particularly, the invention relates to a marking determining method and apparatus for determining a marking position and whether a marking pattern is proper or not.
2. Description of the Related Art
By using a laser beam, a marking pattern by dots can be formed on the surface of a material to be printed such as a photosensitive material. In the case of printing the material to be printed by using a laser beam, the material to be printed is irradiated with a laser beam oscillated by a laser oscillator and condensed to a spot. It causes thermal deformation or the like on the material to be printed, thereby forming a dot. By a scan of a laser beam, a dot matrix according to a character, a sign, or the like is formed.
On the other hand, in dot marking on the photosensitive material or the like with a laser beam, the laser beam is oscillated only at the time of forming a dot. The laser oscillation is stopped in a space on which the dot marking is not performed, thereby saving energy consumption and suppressing heat generation. In such a manner, occurrence of deterioration in quality such as a change in the photosensitive material as the material to be printed due to heat is prevented.
Consequently, the laser oscillator frequently repeats turn-on/turn-off (oscillation/stop oscillation) in extremely short time.
When the dot marking is performed with the laser beam oscillated by frequently repeating turn-on/turn-off in extremely short time, a marking dropout, marking at a wrong timing, or the like may occur due to a failure in an internal part, noises from the outside, and the like. To prevent such poor printing, an in-line test is required.
One of in-line test methods in a process using a laser beam or the like is a method of measuring temperature by detecting an infrared ray emitted from a laser beam irradiation position on a material to be processed at the time of irradiating the material to be processed such as a material to be printed with a laser beam (refer to, for example, Japanese Patent Application Publication (JP-B) No. 59-50434).
Another method is proposed such that, for example, at a time of opening a hole by using an energy beam such as a laser beam, an infrared ray or a thermo electron generated from a melted material around the processed hole are detected and a process state is determined from the intensity and a change in temporal response or thermo electrons (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 62-77195).
Further, a method of determining a process state at the time of processing a thin resin film such as an enveloping material with a laser by detecting the surface temperature of a material to be processed in an irradiation position of a laser beam has been proposed (refer to, for example, JP-A No. 10-305377).
However, at the time of performing dot marking by using a photosensitive material as a material to be printed and irradiating the photosensitive material with a laser beam, irradiation time of the laser beam for one dot is extremely short. Therefore, at a time of emitting a laser beam, emission energy from the irradiation position is also extremely small. To detect the emission energy from the laser beam irradiation position at a time of emitting the laser beam, a sensor having high sensitivity and responsiveness has to be used.
The sensitivity of a sensor can be improved by enhancing cooling of a sensor element. For this purpose, an increase in size is required for a detecting apparatus and it is difficult to realize it from the viewpoint of cost. A sensor having responsiveness on the order of 10 μsec is not obtained under present circumstances. It is substantially impossible to accurately detect the emission energy from a laser beam irradiation position at a time of emitting the laser beam.
On the other hand, as a marking evaluation method, there is a method of determining whether proper dot marking is performed or not by combining a CCD camera or the like with an imaging process.
However, when light in a visible range is used for a photosensitive material, a problem occurs such that the light deteriorates the product quality of the photosensitive material. Consequently, it cannot but depend on a sampling inspection or the like to determine the marking result when dot marking is performed on a photosensitive material with a laser beam.
In the sampling inspection, however, a marking result is tested after completion of marking of one long photosensitive material. Therefore, a problem occurs such that a large amount of the photosensitive material is wasted when a failure such as a marking drop is detected. When an inspection omission occurs, a problem arises such that a number of defective products are manufactured.